One of the problems with Linux gaming is driver support. You either get closed source drivers for the powerful discrete graphics cards, many of which I have read complaints about, or good drivers for the relatively wimpy embedded graphics chipsets.
That seems to be changing, though. Intel’s embedded graphics are improving rapidly. The current HD 4000 chipset is supposedly fairly decent, and will run current games at reduced settings. And the new Haswell chipset that is coming out next month is supposed to be twice as powerful as HD 4000. And, the drivers are open source and excellent.
My plan for building a Linux SteamBox is as follows: Wait for Haswell to come out. Check the reviews to see if it really does perform well enough. Then buy a blacked-out Dell Inspiron with Haswell once they’re affordable.